1. Field of the Invention
The invention relates to a blood constituent measuring device and method, and more particularly relates to a non-invasive device and method for determining the concentration of oxygen in the blood.
2. Description of Pertinent Background Information
The well-known explosion in electronics technology over the past few decades has found many diverse areas of application. On such area is the monitoring of physiological functions. The present invention relates to such monitoring, and specifically, to the measurement of tissue oxygenation.
Monitoring oxygenation levels is desirable in the more critical areas of the hospital, espcially when a patient is being ventilated by machine. There is potential for mishap, both physiological and mechanical. Foremost examples are patients under anesthesia in the operating room, and patients in intensive/critical care units.
Two forms of electronic monitoring have gained widespread acceptance for the monitoring of oxygenation--transcutaneous monitoring of the partial pressure of oxygen, and optical monitoring of the percent hemoglobin saturation (oximetry).
Transcutaneous monitoring seeks to measure directly the partial pressure of oxygen in the tissues by measuring the oxygen which diffuses through a locally heated area of the skin. An implicit assumption of transcutaneous monitoring is good correlation between the partial pressure of diffused oxygen and the partial pressure of oxygen in the tissues. Thick and fatty skin is the Achilles' heel of this approach.
Oximetry seeks to determine the percentage of available hemoglobin in the red blood cells carrying oxygen to the tissues from the lungs. This percentage is related to the partial pressure of oxygen in the blood by the well established oxygen-disassociation curve. The higher the partial pressure, the greater is the diffusion of oxygen from the capillaries to the tissues. Thus, although oxygen saturation is not a direct measurement of the degree of tissue oxygenation, unless the cardiac output (rate at which the heart pumps blood to the body) is impaired, the two measurements will be strongly correlated.
The oximetry measurement is optical--it essentially measures how red the blood is. As most are aware from common experience, oxyhemoglobin (hemoglobin bound with oxygen) is "redder" than hemoglobin.
The method employed in such measurements is spectrophotometry. Spectrophotometry can determine the relative concentrations of N substances in a mixture by measuring the absorption by the mixture of N wavelengths of light, if the absorptions by the individual substances are sufficiently different. Mathematically, the approach amounts to solving N equations in N variables.
In the blood, hemoglobin and oxyhemoglobin are the primary substances which absorb light in the red and near-infrared region of the spectrum. Thus, two wavelengths of light (typically one red and one near-infrared are employed for maximum discrimination) are required to measure the percentage saturation (oxyhemoglobin as a percentage of total hemoglobin and oxyhemoglobin).
In vitro devices (whose use requires drawing a blood sample for measurement external to the body) have existed for a number of years. More recently, in vivo devices (which perform the measurement in blood in the body) have appeared, but these were invasive, requiring a fiber optic tube to be inserted into the bloodstream. Making a practical non-invasive device which could continuously monitor percent saturation did not await only the electronics revolution, however. There were other practical difficulties, for it is the percent saturation of the arterial blood which correlates to tissue oxygenation, and one aspect of the problem, therefore, is how to measure, non-invasively, the absorption of the arterial blood and exclude the contributions by venous blood, bone, skin, etc. One approach by Wood in the 1940's was to squeeze the earlobe to get a reading of the absorption of everything but blood, and then heat the ear to arterialize the blood which entered when the pressure was taken off. In the 1970's, Hewlett-Packard marketed a device which used eight wavelengths of light in an attempt to account for contributions from the non-blood portions of the earlobe. Use of that device also involved heating the ear to arterialize the blood. Neither of these devices were suitable for use in the operation room or intensive/critical care units: they were too large, expensive and complicated to use.
Newer devices, which are gaining widespread acceptance, are of a type called "pulse oximeters". The principle upon which they are based is simple. The light transmitted through the monitoring site (typically the finger, ear or toe), has a pulsatile component related to the extra blood pumped into the arterial vessels of the monitoring site with each heartbeat. This extra blood is arterial. Therefore, analysis of the pulsatile signal yields the percentage oxygen saturation of the arterial blood.
There is another complication related to the in vivo measurement. Strictly speaking, spectrophotometric analysis is based upon a model wiich includes pure collimated light, the intensity of which is reduced only by aborption by the mixture to be analyzed. The intensity is reduced by an exponential process known as "Beer's Law". Calculations used in in vivo measurement assume this exponential process. In non-invasive pulsatile oximetry, the light is diffused by the tissues being analyzed and the pulsatile signal received is due to scattering by the red blood cells as well as absorption by the hemoglobin and oxyhemoglobin molecules in the arterial vessels.
Fortuitously, it is found that if a "Beer's Law" type relationship is assumed, the coefficients which determine the exponential characteristic can be determined experimentally by measurement over a population of patients. Since a scattering process is involved as well as an absorption process, the coefficients are larger, and yet they are consistent enough over a population to be the basis of a useful device.
Such devices are described in U.S. Pat. Nos. 3,998,550, 4,266,554, 4,407,290 and 4,621,643. All are pulsatile oximeters and differ only by the means in whichthe signals are processed. The device of U.S. Pat. No. 3,998,550 solves the exponential Beer's Law equations by using a logarihmic circuit, while that of U.S. Pat. No. 4,266,554 takes the derivative. U.S. Pat. No. 4,407,290 recognizes that the pulse is sufficiently small to allow linerization of the equations, thus obviating the need to solve exponential equations.
While the above patents illustrate the basic principles upon which pulse oximetry is founded, and are directed to devices which are based upon these principles, all of them fail to focus upon some of the specific difficulties associated with the use of such devices in practice. It is important to recognize that these devices are typically utilized to monitor patients who are not healthy. Thus, these devices must operate under conditions of unstable physiological states and on patients who may have very weak pulses. In addition, these devices must operate from monitoring sites which exhibit a wide variation in light transmission properties.